List Server in ME 20.3 don't add subscribers

Hi all! In the past, the List Server run smooth. But now, I have following problem:

We are using a SQL Databse behind to store the Subscribers. Actually no new subscribers are added to the table, neither if the subscribe via Mail or manually added via the console.

SQL Profiler shows only the Select Statement, if I want to see the subscribers. SELECT [ls_id], [ls_first], [ls_last], [ls_email], [ls_unsubscribed], [ls_company] FROM [newsletter_subscribers] WHERE [ls_unsubscribed] = 0 OR [ls_unsubscribed] is Null

After entering a new Mailaddress and hit the "Add" Button, no SQL-Statement is send to the server so the new Mailaddress is never stored in the table. Eventlog on ME and SQL-Server shows nothing. Maybe I'm missing a file with the statement?